1. Field of the Invention
The present invention relates to an austenitic heat-resistant steel which forms a uniform Al.sub.2 O.sub.3 film on its surface in a high-temperature oxidizing atmosphere, exhibits oxidation resistance and high-temperature corrosion resistance similar to Fe-Cr-Al steels, and also shows sufficient hot workability for permitting roll-breaking on a large production scale and good weldability.
2. Description of Prior Art
Conventional Fe-Cr-Al steels which have been widely used as heating elements form Al.sub.2 O.sub.3 film on their surface in a high-temperature oxidation atmosphere, and show resistance against attacks by sulfur and vanadium, but show defects inherent to ferritic steels, such as poor strength at high temperatures, and therefore their applications have been considerably limited.
Meanwhile, austenitic stainless steels show excellent high-temperature strength and cold workability, but their oxide films show poor resistance against spalling, and these steels have a defect that weight decrease is considerable when subjected to cyclic heatings and erosion.
In efforts to overcome these defects of austenitic heat-resistant steels, it has been proposed to add Al to Ni-Cr steels. For example, Japanese Patent Publication No. Sho 47-11576, British Pat. No. 1,147,574, French Pat. No. 1,555,208, and Japanese Laid-Open Patent Application Nos. Sho 48-30621, Sho 49-23125, Sho 50-24117 and Sho 50-51411 disclose such Al-added Ni-Cr steels as heat resistant steels, and Japanese Patent Publication Nos. Sho 34-2554 and Sho 47-23054, Japanese Laid-Open Patent Application No. Sho 48-13213 and German Pat. No. 2,135,180 disclose similar austenitic steels. Further, Japanese Patent Publication No. Sho 49-32685 discloses a ferrite-austenite heat-resistant steel, and various similar steels made by powder metallurgy and other special methods, as disclosed by Russian Pat. No. 287,316, have also been proposed.
All of the above known steels have Al addition not more than 4.5%, and by the Al addition oxidation resistance has been improved, but no uniform Al.sub.2 O.sub.3 film is stably formed, and the film which is formed is composed mainly of a spinel oxide film of Fe, Ni and Cr, as is formed in ordinary austenitic stainless steels. This film is susceptible to spalling and easily permits permeation of oxygen and nitrogen, so that when the steels are used in a high-temperature oxidizing atmosphere, a thick internal oxide layer is formed in the matrix just below the external oxide film, and further, a substantial amount of crystalline TiN and AlN precipitates below the oxide layer.
For illustration of a typical example, FIG. 2(1) shows a cross-sectional view near the surface of 23Cr-24Ni-2Al steel after oxidation tests.
In the photograph, A represents a spinel oxide film of Fe, Ni and Cr, B represents a metallic layer having mainly Al.sub.2 O.sub.3 distributed therein, and C represents an austenite phase in which AlN (larger precipitates) and TiN (smaller precipitates) are precipitated.
Although the above steels show better oxidation resistance than obtained by ordinary Ni-Cr steels, these steels still have the defect that they are susceptible to weight decrease due to spalling, and when used at high temperatures for a long time, a large amount of AlN precipitates to deteriorate the material quality and the effect of Al addition is thus gradually lost, and at a certain point oxidation progresses abruptly.
In order to overcome the above defect of Al-containing stainless steels, it has been proposed, in U.S. Pat. No. 3,754,898, to add a maximum of 5.5% of Al and a minimum of 0.1% of a rare earth element, such as Y, or an actinide element, so as to form a uniform film of Al.sub.2 O.sub.3, thereby assuring excellent resistance to high-temperature corrosion such as caused by exhaust gases of automobile engines. However, the steel has been proved to afford no practical utility due to the fact that it cannot maintain sufficient hot workability for permitting roll-breaking on a large production scale, because of its relatively high Y content, and also the fact that it shows a high sensitivity to high-temperature cracking during welding, because of its fully austenitic structure.